utf8: make EncodeRune's destination the first argument.

R=r
CC=golang-dev
https://golang.org/cl/3364041
diff --git a/src/pkg/bytes/buffer.go b/src/pkg/bytes/buffer.go
index 6f93869..b4ad95f 100644
--- a/src/pkg/bytes/buffer.go
+++ b/src/pkg/bytes/buffer.go
@@ -172,7 +172,7 @@
 		b.WriteByte(byte(r))
 		return 1, nil
 	}
-	n = utf8.EncodeRune(r, b.runeBytes[0:])
+	n = utf8.EncodeRune(b.runeBytes[0:], r)
 	b.Write(b.runeBytes[0:n])
 	return n, nil
 }